The use of fibrous filter cartridges, for example, in medical and industrial processes, is now commonplace. As conventionally used, high dirt capacity, fibrous filter cartridges are arranged to accept fluid flow either from the outside surface of the filter through the filter to the inside or from the inside to the outside of the filter. In the manufacture of tubular fibrous filter elements, end caps or end caps with gaskets are used to seal the open ends of the tube.
End capping is a well known method of sealing the end of a filter element to prevent bypass and to provide a fit into a housing. It is absolutely essential that the seal between the end cap and the filter be leakproof, since otherwise unfiltered fluid can bypass the filter at this point, contaminating the filtrate. One method for attaching end caps to tubular filter elements is applying an adhesive either to the ends of the tube or to the end caps, fitting the end caps to the tube, and maintaining pressure until a firm bond has been effected. Other methods of end capping involve bonding a plastic or metal cap onto the end of its filter with hot melt glues, epoxy resins, ultrasonics, fusion bonding, friction (spin) welding, etc.
End caps provide structural rigidity to the fibrous tube filter. Thus, it is ordinarily desirable that the seal between the end cap and the tube ends be tight.
When rigid, solid thermoplastic or metal end caps are applied to the filter element, it is often desirable to incorporate a deformable, solid elastomeric gasket to allow for slight differences in element lengths. In this way an effective seal is maintained between the filter cartridge and the housing. With tubular filters it is possible to construct an end cap incorporating a gasket which is bonded directly onto the filter. Such an end cap fills the dual function of sealing the end of the filter element and providing a resilient gasket to the housing. In order to achieve the dual function with a unitary end cap/gasket structure, one approach of the industry has been to turn to foamed polymers such as Volara.RTM. foam, a flexible closed cell radiation cross-linked polyethylene foam.
Foamed polymers provide for deformation and sealing. Foamed polymer end caps/gaskets, however, do not possess a high degree of strength in comparison to solid polymers. Also foamed polymer end caps often do not rebound after exposure to substantial temperature cycles, i.e., they possess low compression deflection.
As noted above, Volara.RTM. is a closed cell polyethylene foam and can readily be bonded to a filter element using, for example, a hot fusion technique; that is, heating the filter and bringing it in contact with the end cap to effect bonding. The technique has disadvantages in that the Volara.RTM. sets permanently and, therefore, the seal can be lost-if exposed to successive high and low temperature cycles.
A heat fusion technique can also be employed where the filter cartridge itself is heated and pressed into a foamed thermoplastic end cap. A first disadvantage with this technique is that directly heating the filter cartridge may impair the structural integrity and filtration capacity of the resulting capped filter. Polymeric microfibers generally exhibit fiber pull back upon direct application of heat to the filter. The fibrous mass shrinks due conformational heat instability of the component fibers. A second disadvantage relates to the physical characteristics of the foamed end caps themselves. The foamed end caps do not provide the structural strength or tear resistance of a solid rubber end cap. In industrial filter applications, foamed polymer end caps/gaskets may tear when subject to substantial forces, e.g., torque. If removed from a housing for checking or maintenance, foamed end caps are not likely to be reusable due to either tearing or permanent compressive deformation.
A high strength, tight seal is very difficult to achieve when a filter element is composed of a lofty, thermoplastic non-woven microfibrous mass. Thermoplastic fibers, especially fibers composed of an amorphous, semi-crystalline, or crystalline thermoplastic possess both low heat conductivity and dimensional heat instability at elevated temperatures. In other words, a mass of interentangled fibers will have a tendency to shrink at elevated temperatures. In the case of amorphous or semi-crystalline fibers, surface tension forces drive a liquefied/softened material toward the geometry possessing the lowest surface energy, i.e., a sphere. Accordingly, when an amorphous fiber is exposed to eat energy sufficient to overcome the surface tension conformational energy, the fiber will tend to pull back and, in an extreme case, "roll up" into a sphere. In the case of more crystalline polymers, dimensional heat instability occurs at temperatures exceeding the crystalline transition temperature of the thermoplastic. Accordingly, in a filter cartridge composed of amorphous or crystalline thermoplastic fibers, heating the fibrous material to a temperature that induces shrinkage of the polymeric fibers results in pull back of the fibrous mass, both radially and, more importantly, axially, from the original dimensions of the filter cartridge. Thus, the structural integrity of the filter may be impaired and its filtering capability may be destroyed when the pull back is sufficient to allow bypass.